It is known to those skilled in the art that a bidirectional expansion screw for orthodontics comprises a central spindle, provided with counter-rotating threads on the two sides, and with a central, so-called "actuating" portion of larger diameter allowing the same spindle to be rotated by means of a pin-like tool able to be inserted into suitable recesses or holes formed on the surface thereof, said spindle being usually associated to two cylindrical and parallel rods connected to each other by metal blocks having longitudinal holes into which the same rods are positioned therethrough so as to form a guide assembly for driving the blocks together with the corresponding separate portions of an orthodontic plate or prothesis, as described later on. Formed in each of the blocks which connect said guide rods, is a threaded hole able to receive a corresponding side of the central spindle--as schematically illustrated in FIG. 1E of the attached drawings wherein the letter F indicates the threading and B the block in its whole--so that the rotation of the same spindle will determine the straight displacement of the blocks in opposite directions. Since each block is embedded into a corresponding element of the orthodontic plate, the movement of the blocks brings about the corresponding movement of the two plate elements and, therefore, in order to move these elements close to or away from each other, it is sufficient to turn the spindle by means of the suitable tool.
Based on the same principle is the operation of unidirectional, three-dimensional and fan-like openable expansion screws.
Expansion screws for orthodontics are also known from IT 1163244, U.S. Pat. No. 5,472,344 and GB 641139 documents.
One drawback in the manufacturing of orthodontic expansion screws of the type above described, lies in the fact that, to ensure the necessary stability and functionality of the screw connection between the central spindle and each block of the guide assembly, the construction accuracy is crucial, a factor which weighs heavily on the manufacturing costs because of the qualified labour and precision machines that must be relied on. Moreover, when assembling the various parts of the screw, it is necessary to screw every threaded side or shank of the central spindle into the female screw of the corresponding block of the guide assembly, which brings about a longer manufacturing time in the whole. In addition to this, there is the fact that each block must have a threading corresponding to that of the spindle part to be received and, accordingly, the blocks must be threaded oppositely.
A further drawback to be found especially with the use of this type of orthodontic screw, is related to the inevitable operational clearance of the screw connecting parts, so that the insufficient friction involved between the device's threaded parts in contact with each other may cause spontaneous rotations of the spindle, that is, a displacement of the blocks from their optimal position, especially when considering the strong reaction exerted by the members of the masticatory apparatus to which the prothesis elements are hooked up, and which has a natural tendency to displace the same prothesis from its proper operational position.